This invention is directed to an analytical balance for electronically measuring an unknown mass and digitally displaying the results. It is more specifically directed to a substitution type analytical balance in which the gimbal or substitution weights are selectively removed automatically in direct response to a digital input signal converted from the analogue output of the beam feedback and null restoring circuit.
In the past, it has been customary practice to provide analytical balance mechanisms wherein displacement of a balanced beam is sensed and converted to an electrical control signal. This control signal is applied as a null restoring force through a coil to maintain the beam in its null reference plane. In some cases the amplitude of the control signal developed for restoring the beam is read directly by a suitable display device for showing the weight of the unknown mass.
An example of this type of device is described in U.S. Pat. No. 3,604,525, entitled "Automatic Top-Loading Weigh Apparatus with Electronic Measuring and Recording Circuit", issued Sept. 14, 1971 to the assignee of the present invention. A differential capacitance value is developed by sensing the vertical displacement of an unbalanced beam. This capacitance value through a capacitance bridge circuit is differentially measured by the monitor and control circuit, converted to a variable current output and is applied to an electromagnetic coil to return the beam to its null position. A resultant voltage is developed in direct proportion to the current required to return the beam to its null position. This voltage representing the unknown weight is then displayed by means of a digital voltmeter directly calibrated to read in units of weight.
Another common type of analytical balance which is presently used to provide increased accuracy is known as the substitution type balance. In this balance a horizontal beam is pivotally mounted on a knife edge with a counterweight provided at one end and a weigh pan supported from a gimbal assembly arranged at the opposite end. A plurality of calibrated weights which total the maximum weighing capacity of the balance are suspended from the gimbal assembly. The combined weight of the gimbal assembly, weigh pan and substitution calibrated weights equal the counterweight and keeps the beam balanced in its null reference plane. With an unknown mass placed in the weigh pan, the calibrated weights are systematically removed through a series of mechanical cams and levers until the beam is returned to its null or balanced condition. The total weight of the calibrated substitution weights which were removed to accomplish this balance is equivalent to the unknown mass placed in the weigh pan.
One of the major problems encountered with the substitution type balance, in which the weights are mechanically removed from the gimbal assembly, is the inherent lag and backlash that is present in this type of mechanism. With the use of the cams and levers it is necessary to remove the weights one at a time in sequence which sometimes applies perturbations to the beam which additionally increases the time required to make an accurate measurement.
In order to eliminate this type of problem and to greatly improve the accuracies that can be obtained, it is an object of the present invention to combine the attributes of the substitution balance with the electronic circuitry, utilizing both the capacitive feeback sensors and beam null restoring force coil.
It is another object of the present invention to provide a more rapidly responding system in order to reduce the time required for obtaining readings with a substitution type analytical balance.
Another object of the present invention is to provide a system for automatically and simultaneously removing selected weights from a substitution type balance to allow rapid and accurate weighing of an unknown mass to an accuracy of at least plus or minus 1/10 milligram.
It is a still further object of the present invention to provide an analytical balance in which the output signal for the beam null restoring coil is converted to a digital signal which can be more easily displayed and retained in memory for a later use.
It is a still further object of the present invention to provide an electronic tare circuit for an analytical balance in which the electronic output signal which is proportional to the unknown weight can be stored in a cumulative memory circuit while the display is returned to zero reference for canceling out the weight of a container or a previously weighed mass.